The overall objective of the proposed research is to reduce the amount of energy required for ventricular defibrillation, thereby permitting significant size reductions in an implantable cardioverter defibrillator without compromising defibrillation safety margins. This will be attempted by a Systems approach that includes optimization of the biphasic waveform, specifically targeting the use of small capacitors with values less than 95microF; identifying the most efficacious electrode locations and current pathways; and studying the effects of shock timing on defibrillation threshold. The efficacy of small capacitor biphasic waveforms with varying durations and tilts in combination with high voltage changes at phase reversal will be compared in anesthetized pigs. The influence of electrode size, shape, and location on current distribution and voltage gradients in the heart will be examined using a realistic finite element model of the human thorax. These results will form the basis for additional electrode placement and current pathway studies in humans. Since small capacitors have the advantage of delivering therapy earlier due to faster charging times, we will investigate whether the early delivery of a defibrillation shock as well as the timing of the shock within the fibrillation cycle would provide additional reductions in defibrillation threshold. PROPOSED COMMERCIAL APPLICATION: Proposed commercial application not available.